Sensitization of photoconductive copysheets



United States Patent 3,146,101 SENSITIZATIQN 0F PHQTOCGNDUCTIVECOPYSHEETS Byron W. Neher, Hudson, Wis, assignor to Minnesota Mining andManufacturing Company, St. Paul, Minn., a corporation of Delaware NoDrawing. Filed May 31, 1962, Ser. No. 198,771 Claims. (Cl. 96-1) Thisinvention relates to sensitizing photoconductive copysheets and aprocess for the preparation thereof. In one aspect, this inventionrelates to photoconductive zinc oxide copysheets having improvedspectral sensitivity.

A recently developed image reproduction process involveselectrolytically developing permanent and visible images on suitable,strongly photoconductive copysheets after exposure to light images. Thismethod, described more fully in United States Patent Number 3,010,883,includes the electrolysis of an electrolytic developer and particularlythe selective electrodeposition of a metallic or other visibly distinctcoating at the exposed portions of the photosensitive surface, e.g. byelectrolytic reduction. Strongly photoconductive copysheets suitable forthe above method are described in United States Patent Number 3,010,884.To improve the sensitivity of such photoconductive copysheets variousdye sensitizers, such as acridine orange, have been suggested. However,such dye sensitizers have frequently been strongly colored materials andhave tended to discolor the surface of the photoconductive copysheetsand detract from the quality of the image produced thereon. In certaininstances, only extremely minute quantities of dye sensitizer could betolerated because of discoloration. It has been desired, therefore, tofind a sensitizing material which is relatively colorless and which cantherefore be incorporated into a white photoconductive zinc oxidecopysheet at higher levels of concentration to obtain improved spectralresponse.

It is therefore an object of this invention to provide stronglyphotoconductive copysheets having improved spectral responsecharacteristics.

It is another object of this invention to provide a sensitized zincoxide copysheet having relatively high concentration levels of opticalsensitizer Without substantially affecting the visual color of thecopysheet.

It is still another object of this invention to provide a process forthe preparation of photoconductive copysheets having improved spectralresponse.

In accordance with this invention photoconductive copysheets havingenhanced spectral sensitivity are obtained by incorporating from about 1to about 2,000 parts by Weight of an amino coumarin compound per 40,000parts by weight of photoconductive material in the photoconductive layerof the copysheet. The position of the amino group on the coumarinnucleus has not been found to be critical for sensitization, althoughvariations in the sensitization can be expected depending on theparticular amino coumarin selected. Similarly, the amino group can bearhydrogen, alkyl, alkaryl, or aryl substituents, or the amino nitrogenatom can constitute part of a heterocyclic ring, e.g. piperidine,morpholine, thiomorpholine, piperazine, etc. Illustrative amino coumarincompounds are 3-amino coumarin, 6-amino coumarin, 4-methyl-7-dimethylamino coumarin, 4-methyl-7-diethylamino coumarin, etc.The presence of at least one amino substituent on the coumarin nucleusis essential to obtain the desired sensitization, and amino-freecoumarins, such as 3-methyl coumarin, 4-methyl-7-hydroxy coumarin and6-methyl coumarin have displayed essentially no sensitizing effect onphotoconductive zinc oxide. The preferred amino coumarin sensitizers arerelatively colorless as visually observed in 0.1% solution in methylethyl ketone.

The amino coumarin compounds of this invention may be incorporated intothe photoconductive portions of the copysheet in a variety of Ways. Ithas been found to be particularly useful to add the amino coumarin insolution in an organic solvent, such as methyl ethyl ketone, to themixture of photoconductive material and organic polymeric binder duringthe mixing thereof, preferably during the latter stages of the mixingoperation. When the photoconductor and binder are ball milled for anextended period of time, it is preferable to avoid excessive ballmilling in the presence of the amino coumarin sensitizer. When thephotoconductor and binder material are mixed in the presence of asolvent, such as toluene and methyl ethyl ketone, the amino coumarin maybe added directly to the mix as a dry powder and the resultant admixtureblended. As mentioned earlier, from 1 to 2,000 parts by Weight of theamino coumarin, preferably from 10 to about 1,000 parts by weight, per40,000 parts by Weight of photoconductive material is employed. Zincoxide is preferred as the photoconductive material and is particularlydesirable for the preparation of a white copysheet. The sensitizedadmixture of photoconductor and binder in an organic solvent is thencoated onto a clean electrically conductive backing, e.g. aluminum foilor laminate, as more fully described in United States Patent Number3,010,884.

The increased sensitivity observed with the use of the amino coumarinderivatives of this invention is most particularly evident in the blueregion of the light spectrum, especially at about 425 angstroms. Sincethe sensitizing effect appears to be additive, i.e. a red sensitizerused in conjunction with the amino coumarin tends to provide aphotoconductive coating with enhanced sensitivity to both red and bluelight, other sensitizers and supersensitizers can be used in conjunctionwith these coumarin derivatives. Moreover, since the amino coumarinsensitizers do not tend to adversely discolor the photoconductivecoating, a desirably whiter copysheet can be prepared.

The following examples will illustrate the invention and are notintended to restrict the scope thereof.

Example I An admixture containing 42 weight percent of French processzinc oxide powder (U.S.P. 12), 35 Weight percent of a 30% solution ofbutadiene-styrene copolymer (30:70 mol ratio) in toluene, and 23 weightpercent of methyl ethyl ketone, was ball milled in a pint jar withporcelain balls for four hours and coated onto the aluminum surface ofaluminum laminate paper. The coating was made with a 4-mil Wet coatingthickness. A second mix was prepared in similar manner, with theaddition of 0.042 gram of 4-methyl-7-diethylamino coumarin per grams ofmix before the end of the ball milling operation. This second mix wassimilarly coated onto aluminum laminate. Using a 35 mm. slide containinggray scales and primary color filters in a 200 watt incandescent lightprojector (f3.5, 50 mm. lens) and an enlargement to 5.5 x 8 inches,sensitivity tests were conducted on both coated samples. After standardexposures, the samples were cathodically electrolyzed by sponging withan anode containing sponge holding an aqueous solution of 10% NiCl -6H Oand 5% Na S O thereby developing a dark nickel image on the light struckareas. A marked increase in sensitivity to blue and white light wasobserved in the second sample containing the amino coumarin sensitizercompared to the first sample.

Example 11 The procedure of Example I was repeated, using 1 gram of4-methyl-7-dimethylamino coumarin per 100 grams of mix. An even greaterimprovement in the sensitivity to blue and white light than obtained inExample I was observed. The use of 4-methyl-7-hydroxy coumarin insteadof 4-methyl-7-diethylamino coumarin did not produce any significanctincrease in sensitivity to blue and white light.

Example Ill Following the same procedure as in Example II, 1 gram of4-methyl-7-dimethylamino coumarin was added per 100 grams of mix.Significant increase in sensitivity to blue and white light was noted.Neither 6-methyl coumarin nor 3-methyl coumarin showed any significantimprovement in white light sensitivity.

Example 1V Using the same procedure as in Example II, 1 gram of 6-aminocoumarin was added per 100 grams of mix. The sensitivity to blue andwhite light was markedly increased. 3-amino coumarin also providedimproved sensitivity in the blue portion of the light spectrum.

Various other embodiments of the present invention will be apparent tothose skilled in the art without departing from the scope thereof.

I claim:

1. In a copysheet having a photoconductive zinc oxide therein theimprovement which comprises from about 1 to about 2,000 parts by weightof an amino coumarin per 40,000 parts of said photoconductor, said aminocoumarin being intimately admixed with said photoconductor.

2. In a copysheet having a photoconductive zinc oxide layer theimprovement which comprises an amino coumarin in said photoconductivelayer in a quantity sufficient to increase the sensitivity thereof.

3. In a copysheet having a photoconductive zinc oxide layer theimprovement which comprises 4-methyl-7-diethylamino coumarin in saidphotoconductive layer in a quantity sufficient to increase the bluelight sensitivity thereof.

4. In a copysheet having a photoconductive Zinc oxide layer theimprovement which comprises 4-methyl-7-dimethylamino coumarin in saidphotoconductive layer in a quantity sulficient to increase the bluelight sensitivity thereof.

5. In a copysheet having a photoconductive zinc oxide layer theimprovement which comprises 6-amino coumarin in said photoconductivelayer in a quantity sufficient to increase the blue light sensitivitythereof.

6. In a copysheet having a photoconductive Zinc oxide layer theimprovement which comprises 3-amino-coumarin in said photoconductivelayer in a quantity sufiicient to increase the blue light sensitivitythereof.

7. A photoconductive copysheet having a layer comprising photoconductivezinc oxide material and, intimately admixed therewith, an amino coumarinas a sensitlzer.

8. A photoconductive zinc oxide copysheet having a light sensitive layercomprising a photoconductive powder and, in intimate associationtherewith, between about 1 and about 2,000 parts by weight of an aminocoumarin sensitizer per 40,000 parts of said photoconductive powder.

9. The photoconductive copysheet of claim 8 wherein the amount of saidamino coumarin sensitizer is insufficient to substantially alter thevisual color of the copysheet.

10. A photoconductive copysheet having an electrically conductivesubstrate and a light sensitive layer comprising a photoconductive zincoxide powder and, in intimate association therewith, an amino-coumarinin a quantity sufficient to increase the blue light sensitivity thereofwithout substantially altering the color of said light sensitive layer.

References Cited in the file of this patent UNITED STATES PATENTS2,937,944 Van Dorn et al. May 24, 1960 2,956,027 Thompson Oct. 11, 19603,037,861 Hoegel et al. June 5, 1962 FOREIGN PATENTS 672,741 GreatBritain May 28, 1952 215,294 Austria Oct. 15, 1960 OTHER REFERENCESMees: The Theory of the Photographic Process, Macr millan (1954), pages131-4. 1)

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 146,101 August 25, 1964 Byron W. Neher It is hereby certified. that erroragbears in the above numbered patent requiring correction and that thesa (1 Letters Patent should read as corrected below.

Column 4, line 13, strike out "zinc oxide and insert the same after"photoconductive", in line 14, same column 4.

Signed and sealed this 5th day of January 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BREN ER At testing Officer Commissioner ofPatents

1. IN A COPYSHEET HAVING A PHOTOCONDUCTIVE ZINC OXIDE THEREIN THEIMPROVEMENT WHICH COMPRISES FROM ABOUT 1 TO ABOUT 2,000 PARTS BY WEIGHTOF AN AMINO COUMARIN PER 40,000 PARTS OF SAID PHOTOCONDUCTOR, SAID AMINOCOUMARIN BEING INTIMATELY ADMIXED WITH SAID PHOTOCONDUCTOR.